The present invention generally relates to a wireless communication technique, and more particularly, to a wireless communication apparatus and method using an array antenna.
An adaptive array antenna technique is often used in wireless communication systems. In an adaptive array antenna, multiple dipole antennas are arranged in an array, and the weighting of each of the antennas is controlled in an adaptive manner so as to produce the optimum directivity (or beam pattern) for communication with counterpart equipment. In other words, the antenna gain is increased in the direction of desired signals (in which the counterpart equipment is located), while the antenna gain is made small in directions other than the desired direction. This arrangement allows the desired signals to be received, while reducing interfering waves.
The conventional wireless communications systems with the adaptive array technique, such as a third generation mobile system based on Wideband Code Division Multiple Access (WCDMA), are mainly adapted for circuit switching communication, including speech communication. In the circuit switching communication, the number of communication channels and the data rate are the same in the uplink and the downlink (with a symmetric traffic characteristic), and transmission and reception on the communication channels are continuous. The weighting factor is estimated using the least mean square (LMS) algorithm or the recursive least square (RLS) algorithm so as to converge to the optimum value.
In the circuit switching communication, it can be assumed at high probability that the direction of incoming interfering waves (or the direction of the user who is transmitting the interfering waves) on uplink is consistent with the direction of the incoming interfering waves on downlink. Accordingly, the antenna directivity set in receiving signals, or appropriately calibrated directivity, can also be used when transmitting signals. This realizes efficient signal transmission, while reducing interference. By achieving appropriate directivity on both uplink and downlink, the subscriber capacity of the system can be increased. Such an adaptive array antenna technique is described in, for example, S. Tanaka, M. Sawahashi, and F. Adachi, “Pilot Symbol-assisted Decision-directed Coherent Adaptive Array Diversity for DS-CDMA Mobile Radio Reverse Link”, IEICE Trans. Fundamentals, Vol. E80-A, pp. 2445-2454, December 1997, and A. Harada, S. Tanaka, M. Sawahashi, and F. Adachi, “Performance of Adaptive Antenna Array Diversity Transmitter for W-CDMA Forward Link”, Proc. PIMRC99, pp. 1134-1138, Osaka, Japan, September 1999.
However, the wireless communication systems are currently shifting from circuit switching type to packet type, taking into account the compatibility with the Internet networks. In packet communication systems, burst transmission of communication channels is performed. Transmission and reception on the communication channels are discontinuous in response to the presence or the absence of data packets, instead of continuously transmitting or receiving on the communication channels. For this reason, the LMS algorithm or the RLS algorithm used in the conventional communication system to estimate the optimum weighting factors by taking sufficient time on the assumption of continuous arrival of communication channels cannot be applied to the burst transmission occurring in a short period of time.
In packet switching communications, the uplink traffic and the downlink traffic tend to be asymmetric. For example, when a mobile terminal downloads a large volume of data, a mere command is transmitted on the uplink. In contrast, the downlink traffic requires a large number of channels to transmit the requested data to the mobile terminal. Signal transmission may be carried out only on the downlink, while no signal transmission occurs on the uplink. In this case, producing a transmission beam pattern based on the beam pattern of the received signal does not work. In addition, the assumption that the directions of incoming interfering waves are almost the same on uplink and downlink is not appropriate any longer, because of the asymmetric traffic characteristic and difference in frequencies used. Accordingly, it becomes difficult to reduce interfering waves and realize appropriate beam patterns on uplink and downlink (especially, on downlink).
To estimate weighting factors in the current and future wireless communication systems in which asymmetric traffic is expected, multipath fading occurring along with increased transmission rate and widened frequeuncy band, must to be taken into account. As the sampling frequency increases, multipath components, which are not captured in the conventional wireless communication systems, are detected as interfering waves. In this case, M weighting factors are estimated for each of L paths contained in the received signal, and L signal components received at the respective paths are combined for demodulation of the received signal, as disclosed in the above-described publications, where M is the number of antenna elements of the array antenna and L is the number of paths. Since each path is identified with this technique, the receiving performance is improved. This technique is preferable from the viewpoint of accuracy of the demodulated data.
However, because the received signal power obtained from each path is relatively small, the weighting error (which is the offset of the produced beam pattern from the optimum beam pattern due to thermal noise or other factors) becomes large.
Another known technique is to estimate M weighting factors using a composite signal of all the paths, without identifying each of the paths contained in the received signal. Since the combined signal has relatively large power, the weighting error becomes small. However, when the arriving direction varies among the paths greatly, namely, when the angle spread is large, then the receiving performance is degraded because the multiple paths are not distinguished. Because of multipath fading, interfering waves cannot be reduced sufficiently, and a satisfactory beam pattern cannot be produced.